I'm visiting Athens at the end of the summer... high school covered some of the basics about Plato and Aristotle. But I'd be interested to learn more about Greece's democracy. How did they really influence future civilizations?
A good starting point might be Paul Cartledge's Democracy: A Life, which came out just a few months ago. I haven't read the book myself, but Cartledge is a very respected scholar of Classical Greece, and the book seems to fit your requirements perfectly.
The Greek Way by Edith Hamilton